Composition for and method of treating metals as well as the treated product



United States Pate 2,777,785 Patented Jan. 15, 1957 COMPOSITION FOR ANDMETHOD OF TREATING METALS AS WELL AS THE TREATED PRODUCT NoDrawing.ApplicationJuly 30, 1953, Serial-No. 371,427

The present invention relates to metal coating and particularly to suchcoating which resistance of the metal and/or the adhesion of paint,varnish, lacquer, and ,other'organic finishes to the metal.

Among the objects of the present invention is the novel coating methodsas well as the resulting articles and treating baths for effecting suchcoating.

The above, as well as additional objects of the present invention -w'llbe more completely understood from the following description of severalof'it exemplifications'.

According to the present invention, an extremely ef- :fectivecorrosion-resistant .and organic finish-retaining coating is formed onmetals by applying to the surface of ing is efiected. The resultingcoating should weigh at least mgs. per square foot of metal surface topro vide the desired results. Although the reducing agent can be appliedto the film after the chromicacid, it has been found unexpectedlyadvantageous to include the repolyethylene glycols rolled steel againstcorrosion for a considerable periold of In an atmosphere of 99% relativehumidity. at 125 F. black plate coated in accordance'withthepresent Whenuncoated, practically completely corroded and entirely useless before'24 hours of exposure. However, the chronlic acid'bath itself willgradually undergo deterioration, apparently by vreason of the gradualreduction of the ehrornic acid. When a precipitate appears in a bath, itcan no longer be used commercially inasmuch as the precipitate depositson the metal in spots and renders the final coating .non uniform. It hasbeen discovered that the life of the bath is greatly prolonged if thebath also contains a negative catalyst such as a dissolved man- A veryeffective distribution of, the

film 15 obtained at low rolling speeds when the rubber At higher Naturalor other resilient materials such as plasdrying to a temperature above212 F. Above 350 F. very little additional advantage isobtained. Atdrying temperatures of 450 to 550 F. the coating appears to be adverselyaffected, depending upon the particular reducfected as flaming, that is,passing the filmed metal directly through gas flames. Inasmuch asoxidizing actionis preferably sharply restricted during the drying ofthe-present invention, it is better when flaming the metal to passit'through the reducing ,portion'of thegas flame rather thantheoxidizing portion. Flaming is generally bet-terthan-the use of an airoven, because the flaming effects a considerable portion of its heatingby radiation so At drying temperatures. of 212 aqueous coating film isconverted to coating in about two to five-seconds.

is a very light grayish, and slightly greenish, more or less transparentlayer that is very adherently held on the metal and does not scratch offvery easily. The folding of a sheet of metal back on itself so as toproduce a sharp crease does not loosen the coatingeither on the insideor outside of the crease.

The corrosion resistance of the coating can be further improved byquenching the coated metal from the drying temperature down at least 25F. in a liquid such as water or oil. This treatment lengthens about 50%the period in which the coated metal can be exposed to a hot humidatmosphere without showing corrosion. liven better corrosion resistanceis obtained by quenching in water containing up to about 1% chromicacid. The lacquer adhesion of the coating is also improved in waterquenching where about or more of chromic acid is dissolved. At lowerconcentrations of chromic acid the lacquer adherence is slightlydiminished.

A particularly efiective coating operation is one that ends with adrying step performed at 250 F. followed by a quenching down to 200 F.in a /z% chromic acid solution in water.

The above type of coating operation is effective with all metalsincluding ferrous metals such as iron, steel, both low carbon as well ashigh carbon, stainless steel, aluminum, copper, nickel, magnesium, tin,Zinc and alloys of these metals with each other. With ferrous metal,however, and particularly readily corrodible forms of this metal, thepresent invention is particularly valuable. A very desirable use of thepresent invention is in the manufacture of metal containers such asthose used for oil, foods including milk as well as dried foods, andchemicals. For this purpose cold-rolled sheet or strip carbon steelcontaining from about 0.05 to 0.2% carbon is coated in the mannerpointed out above and then formed into a container such as the usualcrimped seam food can, after which a layer of lacquer can be applied tothe outside of the can surface. An application of the lacquer coating inthis manner helps to seal the crimped joints against leakage. Ifdesired, however, the lacquer can be applied to the inside as well as tothe outside, or alternatively only to the crimped areas, preferablybefore the crimping is effected so that the lacquer has a gasket-likeleak-reducing effect on the joint. Such leakage reduction can also beprovided with other, plastic materials such as resins that are not inlacquer form.

In the coating of ferrous metals additional improvements can be made inaccordance with the present invention by first etching the metal with anitric acid solution in water before the chromic acid treatment. Theetching should be as uniform as practicable and a metal removal of atleast about 100 milligrams per square foot of metal surface is veryeffective. There appears to be no upper limit for the amount of metalremoval that will provide the desired increase in corrosion resistanceand organic film adhesion. However, an etch of more than about 1000milligrams per square foot will tend to leave a roughened surface thatdoes not make a good appearance. Furthermore, in the interest of economythe etch is preferably kept down to not more than about 450 milligramsper square foot. In use the etching bath accumulates a relatively largeamount of dissolved iron and may then deposit a rust-like coating on themetal, especially when the dissolved iron is in the ferric form. Beforethis build up, however, the nitric acid treatmentcan be at any desiredconcentra tion, time, and temperature so long as it effects a uniformmetal removal. A bath temperature of about 120 F. appears to be apractical upper limit for this ose. An etching bath that deposits arust-like coating can be used providing this coating is not sotenaciously held that it cannot be brushed otf as by means of felt pads.or fiber-bristle brushes.

The effect of etching appears to be to form an invisible amount ofreactive iron oxide on the surface of the metal, and in the subsequentchromic acid solution the iron oxide appears to react with the chromicacid before as well as during reduction of the chromic acid to form thedesired coating.

The best types of final coating. in accordance with the presentinvention are actually. water repellent. An appreciable drying operationof at least five seconds duration at 250 F. to 300 F. forms such acoating. More vigorous drying, as at temperatures around 500 F.,produces a coating that is not quite fully water repellent but stillhighly suitable forcorrosion resistance and lacquer adherence. Drying attemperatures below about 500 F. gives more immediate ,water repellancyif the dry coating is kept exposed to th'e 'atmosphere for at least ahalf minute before the coated metal is coiled or otherwise covered.There is very little difference in the corrosion resistance and organicfilm adhesion between the partially and completely water repellentcoating. In fact the partially water repellent coating will nponstanding gradually become completely water repellent.

The time necessary to subject the coated metal to the above drying orcuring conditions varies with the thickness of the metal, Where platesor other relatively thick metals are used more heat is generallyrequired to reach the desired temperature.

Maximum corrosion resistance is obtained when the least possible watersoluble materials are included in the final coating. For this reason thenitric acid etching should preferably be followed by a rinse to wash offsurplus nitric acid as well as any water soluble iron compounds. Alsothe chromic acid bath should be substantially free of substances thatwill not be converted into completely water insoluble form by theprocess. The best reducing agents are accordingly those that do notintroduce water soluble material to any appreciable degree into thefinal coating. Although phenol and potassium iodide are listed above assuitable reducing agents the final coatings they produce are not quiteas protective as the coating produced with other reducing agents.Similarly any reducing agent is best used in an amount that isstoichiometrically no larger than is oxidizable by'the chromic acid tobe reduced. It is possible to reduce only about half of the chromicacid. However, coatings with up to about 5% of water soluble materialare still of outstanding utility in accordance with the presentinvention, as for example to make containers for dry foods, oils orsimilar materials.

The nitric acid treatment is sometimes effected under conditions inwhich ferrous metal becomes passive. When this takes place, the etchingeither does not occur or is only spotty over the surface of metal beingtreated. The passivating effect is serious with high concentrations ofnitric acid butwill be troublesome even with lower concentrations ofthis acid where the etching bath has picked up a sizeable amount ofdissolved iron.

According to the present invention, the etching bath is advisedlypreceded by an activating treatment. Examples of such treatments are thecontacting of the metal with a liquid non-oxidizing etchant such assolutions of acids including:

HCl

H3PO3 plus a small amount of HNOa Citric acid Acetic acidTrichloroacetic acid Tartaric acid and even tannic acid A clip in anaqueous solution of any of the above materials at concentrations as lowas 4 by weight will so aifect the metal that it will not show anypassivity to introduce water soluble materials into the dual coat- F. orat 70 F. in a NaHSOs in water will prevent passivity. A 2.3% solution ofHCl in water will also produce the desired results with a /2 second dipat 35 F. If the activation step causes etching of the metal. ment can beperformed either before or during the nitric acid treatment.

Where the metal to be coated is not perfectly clean, it should besubjected to a careful cleaning operation before the nitric acid etch,or Where no such etch is used, before contact cleaning can be of anykind such as degreasing with or- Without limiting the invention in anyway, the following specific examples are given of elfective coatingoperations.

Example 1 A strip .of full bright finish sheet steel (black plate) tenmils thick is subjected to the following treatment:

A. Cathodically clean in an aqueous solution containing 16 grams of KOHper liter at 140-160 F. at'a current density of i6 amperes per squarefoot of cathode for 10 seconds.

B. Cold water rinse.

C. Clean anodically in an aqueous solution containing 16 grams of KOHper liter at Mil-160 F. at a current density of 16 amperes per squarefoot of anode for 10 seconds.

D. Cold water rinse.

E. Flood with 2.3 percent HNO; in water at 80 F. for

five seconds.

F. Cold water rinse (flood).

G. Flood with an aqueous solution containing 3.2 percent CrOg and 2percent glycerine at 80 F. for two seconds.

H. Roll between smooth rubber rolls wetted with the same solution.

I. Dry by flaming both sides of the resulting strip with a reducing gasflame for-twoseconds, the strip reaching a temperature of 250-270 F.-

1. Permit the strip to cool in the Example 2 Same as Example No. 1,except in G an aqueous solution containing 6.5 percent CrOa and 2percent glycerine is used.

Example 3 Same as Example No. 1, except in G an aqueous solutioncontaining 5 percent CrOa and 2 percent cane sugar is used.

Example 4 Same as Example No. 1, except in G an aqueous solutioncontaining 10 percent CrOs and 3 percent cane sugar is used.

Example 5 Same as Example N0. 4, except in I, dry by flaming one side ofthe strip with a reducing gas flame.

The strip produced in Examples 2 to 5 inclusive behaved about the sameas that of Example 1.

Example 6 Same as Example 1 except that between steps I and J, the stripwas quenched in an aqueous solution containing 0.5% CrOa and held at 200F.

Example 7 A sheet of No. 7 finish black plate 11 mils thick is treatedas follows:

A. Immerse in an aqueous solution containing 3.0%

HNO: for 5 seconds at 75 F.

B. Cold Water rinse.

C. Immerse in an aqueous solution containing 5.0%

CIO: and .5 cane sugar and held at F.

D. Roll through coarse rubber rolls.

E. Dry between infra-red lights for 10 seconds at 250- F. Cool bystanding the sheetin the air.

Example 8 Same as Example No. 7, except in C an aqueous solutioncontaining 2.5% CrOs and .4% glycerine is used.

Example 9 Same as Example No. 7, except in a solution containing 5%CrO3, 1.3% glycerine and .5% HaSOris used.

Example 10 Same as Example No. 7, except in C a solution containing 5%CIOs and 4% manm'tol is used.

Example '11 Same as Example No. 7, exceptin C a solution containing 5%CIOs and 2% carbowax 1500 is used. bowax 1500 is a high molecularpolyethylene glycol, formula CI-I2OH(CH2OCH2)JCCH2QH where xis about30.)

- Example 12 C a solution conglycerine is used at a tempera- Same asExample No. 7, except in taining CIO: and 1% ture of 180-l90,F.

V I Example 13 Same as Example No. 1 except omit forced drying step I-insteaddry by letting stand in air.

' Example 14 A sheet of 28 gauge full bright finish steel (black plate)is subjected to the following treatment:

A: Treat by -immersing 1n an aqueous solution containing 2.5% HNOa for 5seconds at 70 F.

B. Cold water rinse.

C. Treat by immersing 2.5% CrQa and .7 70 F. Y

D. Roll between coarse rubber rolls.

E. Dry between infra-red lights for 15 seconds the metal temperaturereaching 240 F.

F. Quench in water held at 200 F.

in an aqueous solution containing triethanolamine for 3-5 seconds atExample 15 Same as Example No. 14 except in C an aqueous solution of2.5% CIOs and 2% triethanolamine for 3-5 seeonds at 180-l9 0 F. is used.

Example 16 Same as Example 14 except in C an aqueous solution of 2.5CrOz and 1.5% hydroxylamine sulfate is used.

Example 17 Same as Example No. 14 except in C an aqueous solution of 2.5C1'O3 and 2% phosphorous acid is used.

Example 18 Same as Example 14 except in C an aqueous solution of 2.5%CrOa and 3% potassium iodide is used.

Example 1 9 Same as Example No. 14 except in C an aqueous solution 2.5%C103 and, .5 %,,phenol used.

Example 20 Same as Example No. 14 except in C an aqueous solution of2.5% CIOs and .5% hydroquinone used.

Example 21 Same as Example No. 14 except in C an aqueous solution of 20%CrOs and 10% triethanolamine at a temperature of 130 F. is employed.

Example 22 A sheet of 26 gauge full bright finish steel (black plate) issubjected to the following treatment:

A. Treat by immersing in an aqueous solution containing 2.5% HNO: for 5seconds at 75 F.

B. Cold water rinse.

C. Treat by immersing in an aqueous solution containing 5% C1303 for 3-5seconds at 180 F.

D. Roll between coarse rubber rolls.

E. Dry between infra-red lights for 7 seconds.

F. Hold in fumes from a boiling aqueous solution of formaldehyde (37%)for seconds.

G. Dry between infra-red lights for 10 seconds the sheet temperaturereaching 220 F.

H. Cool by standing in air.

Example 23 Same as Example No. 22 except in step C use a temfumes fromboiling perature of 75 E, and in step F ethyl alcohol are used.

Cit

Example 24 A. Clean 28 gauge black plate cathodically in an aqueoussolution containing 16 grams KOH per liter using a current density of 15amperes per square foot of cathode at 140-160 F. for 10 seconds.

B. Cold water rinse.

C. Clean anodically in an aqueous solution containing 16 grams KOH perliter using a current density of 15 amperes per square foot of cathodeat 140-160 F. for 10 seconds. I

D. Cold water rinse.

E. Flood with anaqueous solution containing 10% CrOz.v

and 3% cane sugar, at F. for 2 seconds.

FL Roll through smooth rubber rolls wetted with the solution mentionedin E.

G. Dry the sheet by flaming both sides of the sheet with reducing gasflames.

H. Cool by standing in air.

Example 25 except in E, a solution conglycerine is used.

Same as Example No. 24, taining 6.5% CrOa and 2% Example 26 The productof this example has a very satisfactory corrosion resistance whereas anidentical metal finished the same process but omitting the in accordancewith reducing agent in step E had an unacceptable corrosion 5% CrOsresistance.

. Example 27 Same as Example No. '26, except that in C" there is used a5% citric acid solution in water at F. for

10 seconds instead of the HCl solution.

Example 28 Same as Example No. 26, except that in C there is used a 10%HzSOi solution .in'water at F. for 10 seconds instead of the HClsolution.

Example 29 Same as Example 26, except that in C there is used an aqueoussolution having 3% ferric sulfate at 106 F. for 5 seconds instead of theHCl solution.

Example 30 Same as Example No. 26, except that in C there is used anaqueous solution having 3% oxalic acid at 160 F. for 10 seconds insteadof the HCl solution.

Example 31 I Same as Example No. 26, except that in (2" there is used anaqueous solution of 39% NaOH and 2% NaNOs at 280 F. for 15 minutes inlieu of the HCl treatment.

v Example 32 I Treating of aluminum metal, Type 38 /2 H:

A. Immerse 25 mil thick sheet aluminum type 3S- /2 H in an aqueoussolution containing 2% KOH for 1% minutes at F. H B. Water rinse at 70F.

C. Immerse the rinsed metalgin an aqueous solution containing 10% HNOsfor 1-minute at130"-;F.

D. Water rinse at 70 F.

E. Immerse the resulting metalinan aqueous solution containing 2.5% CrOsand 2% HsPOs for 5 seconds at 75 F.

F. Dry between infra-red lights for 1% minntesthemetal reaching 200 F.

G. Cool by standing in air.

The product had a thin grey coating and was not attacked during 60seconds immersion a solution containing 1% KOH at 85 F. A product madethe same way except that the reducing agent'in step E is omitted, isattacked at the end of 25 seconds under the same conditions. I

Example 33 A. A strip of 28 gauge black plate having a brightelectrolytic tin coating weighing 300'milligrams per square foot ofmetal surface, is cleaned anodically in an aqueous solution containing1% NazQOa at 70 F. for

15-20 seconds using a current density of 1 5.arnperes per square foot ofanode.

B. Water rinse at 70 F. C. Immerse in an aqueous solution containing0.2%

H2504 for l to 2 seconds at 75 F.

D. Water rinse at 70 F. E. Dip in an aqueous bath containing 20% .CIOsand triethanolamine at 70 F. for5 seconds.

F. Roll between coarse rubber rolls. G. Dry between infra-red lights for1 minute, the metal temperature reaching 250-300 F.

H. Cool by standing in air.

A water repellent, insoluble film showing very good lacquer adherenceand corrosion preventionis produced by this process.

Example 34 28 gauge black plate having an electrogalvanized-coatingWeighing 200 milligrams per, square foot ofsurface is treated asfollows.

A. Swab with trichloroethylene.

B. Rinse for seconds in an aqueous solution containing 4 ounces pergallon of trisodium phosphate at 180 F.

C. Water rinse at 70 F.

D. Immerse at 70 F. in an aqueous solution containing 10% CrOs and 3%triethanolaminefor 5 seconds.

E. Roll between coarse rubber rollers.

F. Dry between infra-red lights for 1 G. Cool by standing in air.

The resulting article has a water repellent, insoluble film of excellentprotective properties.

minutes at Examp e 35 Same as Example 34 except that the final cooling,instead of by standing in air, is elfected by queue in tap watercontaining 0.1% CrQs and held at 200 F., the

contact time in the quenching solution being 2 seconds. Erample 36 Theblack plate of Example lis treatedas follows;

E. Brushed to assure that adherent reaction products orcontaminants areremoved. F. Passed through a water solution containing 236% CrOs, 0.9%sucrose, 0.1% KMnQr-atflfi t Fr, and thenpassedbetween smooth rubberrollers.

The coated metals of Examples 35 and 36 are even more corrosionresistant and show better organic finish adhesion than the products ofExamples 34 and 1, respectively.

Example 37 Same as Example 24 except that instead of the black plate, asheet of 26 gauge A. I. S. I. type 430 stainless steel is used, thechromic acid bath containing 3% CrOa and 1 /2% cane sugar and the curingbeing elfected by a. 5 second pass through an incandescent gas burnerassembly, the metal reac ga temperature of 300 F.

The final coated metal shows an excellent retention of paint, lacquerand varnish, and is well protected against pitting in salty atmospheres.The above coating of the 430 stainless steel is further improved by apreliminary etching as in an aqueous solution containing by Weight 8%HNO: and l /z% HP at F. forZminutes. This etching is effected justbefore the treatment With the CrOa-cane sugar solution.

In the above examples all percentages are by weight. Any of the productsmade in accordance with these exarrples has a very satisfactorycorrosion resistance and lacquer adhesion. Similar very effectiveresults are obtained when other metals as listed above are treated withthe present process. All manners of various metals including cold-rolledand hot-rolled steels and irons also show the advantages of thisinvention.

The coatings and coating processes described above are highly suited forprotection against corrosion as Well as to improve the adhesion ofpaints, lacquers, varnishes, etc. In some cases, however, these coatingsmay show a tendency to brittleness caused by sudden impact, as forexample when a coated metal is subjected to a high speed stampingoperation. This tendency to brittleness can be considerably reduced byusing coatings Weighing not more than about 50 milligrams per squarefoot of metal surface, as Well as by decreasing the ratio ofreducingagent to the chromic acid. A good ratio for this purpose isabout the lower limit of the range suitable to form a coating that givesthe effective corrosion resistance. By way of example, an aqueoussolution containing 3% chromic acid and 0.75% sucrose produces a coatingthat is extremely resistant to mechanical shocks and can be stampedatany desired stamping speed without showing significant damage.

A feature of the present invention is that the coatings it produces areall of relatively low cost and therefore can be Widely used with aminimum of difnculty. The cost of the coating can even be furtherreduced by recovering nitric acid from the etching bath after it becomeexhausted. Thi exhausted bath generally contains a relatively largeamount (about 50 to 60 grams per liter) of ferrous iron as Well as asmall amount, usually less than 10 grams per liter, of ferric iron. Theexhausted bath also usually contains as much as 4 or more grams perliter of HNOa, as well as additional nitrate ion content correspondingto that of the ferrous and ferric iron.

Much if .not all of the nitrate ion content of the exhaustedbath can bedirectly-recovered for reuse by merely diminishing the dissolved ironconcentration as by means of.cation exchange treatment. Thus asulfonated polystyrene type cation exchange resin having activatinggroups which e essentially,entirely +SOsl-I groups, such as those i 11through this bed. The solution emerging from the iron exchanger has aniron content about A to li that of the incoming solution, after acontact time of only about one to two minutes. Much if not all oftheferric iron in the incoming solution appears to be unafiected and iscarried right through in the ion exchange treated solution. On. reusesome of the ferric iron remaining in the reused bath appears to becomeconverted to ferrous iron, so that the build-up of ferric iron is not asserious as would otherwise be expected. The ion exchange resin. after ithas become saturated, that is lost much of its ability to removedissolved iron, can be regenerated with sulfuric acid or other strongacid of any desired concentration such as for example a by weightsolution in water.

For uninterrupted operation, it is preferred to have two batches of ionexchange beds, one through which the etching bath is passed, while theother is being regenerated. As soon as a batch of iron exchange resinbecomes exhausted, a simple switching operation can then be used toimmediately switch the etching solution to the freshly regeneratedbatch, and to begin the regeneration of the exhausted bath.

it is not necessary to wait until the etching bath is completelyexhausted before it is subjected to a recovery operation. Thus therecovery can be eifected continuously while the etching bath is beingused, as by continuously circulating the etching bath through a batch ofthe ion exchange material. in this way the etching bath can bemaintained much more uniform in composition so that less supervision andcontrol is needed.

Furthermore, by withdrawing the excess dissolved iron before it hasbuilt-up to a very high concentration, the increase in nitric acidcontent produced by the ion exchange is kept to a minimum and any attackof ion exchange material by the resin is also minimized.

The ferric iron content of the etching bath appears to be present assome type of complex ion that is not removed by a cation exchangematerial. However, by withdrawing the ferrous iron through an exchangebed before the dissolved iron content gets too high, the total ferriciron build-up in the bath is kept to very small amounts.

An additional technique for recovering usable values from the exhaustedetching bath is merely boiling the exhausted bath. Free nitric acid isthereby distilled ofi and at the same time any ferric nitrate ishydrolyzed to give off the nitrate iron bound in this fashion. After allpossible nitric acid has been recovered in this manner, the residue canbe oxidized as by blowing with air to convert the ferrous iron contentto ferric iron. After oxidation, the material can again be boiled todistill ofi further amounts of nitric acid. Alternatively the blowing orsimilar oxidation can be carried out before any distillation, so thatonly one distillation step is needed.

Both of the above recovery techniques can be used together if desired.By way of example, the bath can be rejuvenated with cation exchangeresin until the ferric iron content becomes excessive. At that time thebath may become very viscous and difficult to use, but it can be boiledto distill over all its free nitric acid as well as nitric acid formedby the hydrolysis of its ferric nitrate content. As substantially all ofthe ferrous nitrate can be removed by ion exchange before thisdistillation, substantially all of the original nitric acid will berecovered.

As indicated above the proportion of reducing agent to chromic acid inthe coacting bath is such that the chromic acid is not completelyreduced. Where sucrose is the reducing agent a proportion within therange of from 1 part of chromic acid for 1 part of sucrose, to about 7parts of chromic acid for -1 part of sucrose by weight is suitable. Withtriethanolamine a range of from Z'parts of chromic acid for 1 part ofreducing agent, to about 10 parts of chromic acid for 1 part reducingWith other reducing agents agent by weight is suitable. proportionsgiving about the same results are used.

In general the preliminary etching in an aqueous solution of nitric acidis very effective with plain carbon steels or low alloy steels, thatis,alloys containing not over about 5 to 6% of total alloying ingredients.Steels having higher alloy contents tend to be immune to etching by sucha solution and may require the addition of supplemental etchingmaterials such as the hydrofluoric acid referred to above.

Thepresent application is in part a continuation of copendingapplications, Serial No. 277,286 filed March 18, 1952, and Serial No.278,481 filed March 25, 1952.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope hereof, it is to beunderstood that the invention is not limited to the specific embodimentshereof except as defined in the appended claims.

What is claimed is:

l. A stabilized solution consisting essentially of chromic acid, areducing agent for the chromic acid, and water, the reducing agent beingone that at 80 F. coexists in dissolved condition with the chromic acidfor at least about one day without producing a precipitate, the

solution also containing about 0.01 to 0.5% of a dissolved manganesecompound.

2. The combination of claim -1 in which the manganese compound is KMnOr.

3. The combination of claim 1 in which the manganese I compound isMnCOa.

at least 25 F.

'4. An aqueous metal coating solution containing essentially /2 to 30%CrOz, to 25% of an organic polyalcohol reducing agent, and about 0.01 to0.5% of a dissolved manganese compound.

5. An aqueous solution containing C103, 1.5% of a sugar, and 0.1%KMIIOI.

6. A method for protectively coating ferrous metals which comprisesapplying to the metal an aqueous solution of chromic acid, drying thesolution on the metal by heating the metal to a temperature at least ashigh as 212 F. while reducing at least half but not all the chromic acidwith a reducing agent to form a dried coating weighing at least 10milligrams per square foot of. coated surface, and liquid quenching thedried metal by essentially 3 7. The invention of claim 6 in which thequenching liquid is water.

8. The invention of claim 6 in which the quenching liquid is watercontaining 0.05 to 1 percent CrOa.

9. The invention of claim 6 in which the drying is effected at about 250F. and the quenching liquid is held at about 200 F.

10. The invention of claim 6 in which the reducing agent is an organicpolyalcohol.

11. The product produced by the method of claim 6.

12. The product produced by the method of claim 8.

13. A method for forming a corrosion-resistant, lacquer-adhering surfaceon ferrous metal, which method comprises contacting the metal with anaqueous nitric acid bath having a concentration of from 1 to 20 percentHNOs by weight for a period of from 2 to seconds at a temperature of 60to F., rinsing the contacted metal, applying to the rinsed metal a filmof a bath consisting essentially of water having dissolved in it about/2 to '30 percent CrOs, from about A to 25 percent of a reducing agentfor the chromic acid, said reducing agent being one that co-exists inthe bath with the chromic acid without producing a precipitate at thecontacting temperature for at least one day, and about 0.01 to 0.5percent of a manganese compound, and then drying the filmed metal byheating to cause the reducing agentto be completely oxidized by thechromic acid in the film.

14. The invention of claim 13 in which the reducing h agent is anorganic polyalcohol.

15. The invention ofclaim 13'in which the drying is carried outat atemperature of from 250 to 450 F.

16. The invention not is liquid quenche least 25 F.

References Cited in the file of this patent UNITED STATES PATENTS 13 ofclaim 15 in which the dried prod- 2,315,564 d from the dryingtemperature by at 2,393,663

14 Thompson et a1 Apr. 6, 1943 Thomas et a1 Jan. 29, 1946 Thomas et a1.Jan. 13, 1948 Hempel Dec. 26, 1950 Watson July 10, 19511 FOREIGN PATENTSGreat Britain Jan. 21, 1944 Great Britain Mar. 21, 1947 Great BritainFeb. 3, 1948

1. A STABILIZED SOLUTION CONSISTING ESSENTIALLY OF CHROMIC ACID, AREDUCING AGENT FOR THE CHROMIC ACID, AND WATER, THE REDUCING AGENT BEINGONE THAT AT 80*F. COEXISTS IN DISSOLVED CONDITION WITH THE CHROMIC ACIDFOR ATLEAST ABOUT ONE DAY WITHOUT PRODUCING A PRECIPITATE, THE SOLUTIONALSO CONTAINING ABOUT 0.01 TO 0.5% OF A DISSOLVED MANGANESE COMPOUND.